1. Field of the Invention
The present invention relates to a surface acoustic wave filter utilizing a surface acoustic wave resonator. More particularly, the present invention relates to a surface acoustic wave filter which handles signals having a large electric power in a high frequency region.
2. Description of the Related Art
In recent years, intensive study has been positively conducted on a surface acoustic wave device which utilizes a surface acoustic wave, in order to apply the SAW device to a filter (hereinafter "surface acoustic wave" is abbreviated as SAW). SAW filters have been positively developed with the recent development in mobile communications in which signals of higher frequencies are used.
There are some known methods for configuring a filter using a SAW device for a high frequency band, especially for a band of several hundreds of MHz. Typical known methods include, for example: a method for configuring a filter using a plurality of SAW resonators as described in Japanese Laid-Open Patent Publication No. 52-19044; a method for configuring a filter in which each of input and output interdigital transducers of a SAW resonator is divided into a plurality of portions (such a structure is called an interdigitated interdigital transducer structure) as described in Japanese Laid-Open Patent Publication No. 58-154917; and a method for configuring a filter in which SAW resonators are disposed adjacently and capacitively coupled to each other as described in Japanese Laid-Open Patent Publication No. 3-222512.
SAW filters are required to have a smaller size arid an improved performance, in accordance with the miniaturization of mobile communication apparatus in recent years. In addition, SAW filters are more often used in various portions of the mobile communication apparatus.
Mobile communication apparatus used in a communication system in which signals are simultaneously transmitted and received has a transmitting circuit and a receiving circuit which are connected to each other. A SAW filter may be used as a receiving filter included in the receiving circuit of the communication apparatus having the above-described configuration. However, if the SAW filter is used for such a purpose, a SAW resonator constituting the SAW filter may be deteriorated in a relatively short period of time. This may disadvantageously result in deterioration of the filter characteristics. Accordingly, there exits a problem in that, if the SAW filter is used as a receiving filter in a communication apparatus capable of simultaneously transmitting and receiving signals, desired advantages cannot be attained.